Conventionally, wavelength multiplexing is generally used in an optical transmission system. Wavelength multiplexing is also known as wavelength division multiplexing (WDM), and is an optical transmission technique of multiplex-transmitting plural optical signals of different wavelengths through one optical fiber. When an optical signal is transmitted over a long distance through an optical fiber in a wavelength-division-multiplexing optical transmission system, a loss is generated in a transmission path. Therefore, the optical signal needs to be amplified by installing a repeater between a transmission end and a receiving end. In addition, an optical amplifier is used in the repeater in the wavelength-division-multiplexing optical transmission system to compensate for the loss in the transmission path.
Because the power of transmitted optical signal gives negative influence on a transmission characteristic when the power is too large or too small, each repeater needs to amplify an input optical signal to have appropriate power, and to output the amplified optical signal. Generally, when amplifying a WDM optical signal, an amplification gain is controlled such that the total power of the WDM optical signal becomes at a constant level.
In this case, when the number of wavelengths in wavelength division multiplexing changes, power per one wavelength changes, and negative influence is generated in a transmission characteristic. Therefore, the amplification gain needs to be controlled according to the number of wavelength division multiplexing such that the power per one wavelength becomes constant. As a technique of amplifying the WDM optical signal, there has been proposed a technique of obtaining wavelength number information by counting the number of wavelengths within each repeater (see, for example, Patent Document 1).
According to such a conventional method, transmission and reception of the wavelength number information are not performed between repeaters. Thus, this method has characteristics such that there will be no reception of erroneous wavelength-number information due to noise or wavelength distortion, and the amplification gain can be changed on a real time basis.
Patent Document 1: Japanese Patent Application Laid-open No. H10-303821